1. Field of Invention
This invention relates to pulse transmission systems and in particular to the retiming of serial bit streams to reduce phase distortion (jitter) introduced by transmission via an imperfect transmission medium.
2. Description of the Prior Art
In prior-art devices, the basic timing information was derived from the serial bit stream itself. Retiming was accomplished by controlling the frequency of a local oscillator by the phase difference between the incoming pulse train and the local oscillator output. Thus, phase jitter was reduced by allowing the local oscillator to smooth out the phase differences in the phase of the incoming bit stream.
An improvement over the above-described prior-art technique is disclosed in U.S. Pat. No. 3,093,815. In accordance with the invention therein described, coarse retiming is accomplished in much the same manner as is discussed for the above-described prior-art technique. This coarse timing information was utilized to write the successive elements of a serial bit stream into one of a number of the buffer storage cells in a fixed, cyclic order. At a later time, for example half of the period of a storage cycle, this information was read from the buffer storage elements under the control of a highly stabilized local clock. The clock frequency, and hence the read-out rate, was controlled by a signal representing the average amount of information stored in the buffer storage cells at any particular time. Thus, the local clock rate was made responsive to the average amount of data in the buffer storage cells, to prevent an undue accumulation or depletion from the buffer store.
A principal disadvantage of the patented technique is the complexity and, hence, cost of deriving a control signal which represents the average amount of information stored in the buffer storage cells at any particular instant and in using this signal to control the "local clock" frequency. Such prior-art arrangements employ circuitry which provides for constant write/read comparisons and for frequency adjustment of a voltage-controlled oscillator used as a read clock. It is a purpose of this invention to retain the retiming feature, i.e., phase jitter reduction and fixed phase compensation without the need for constant write/read phase comparison and frequency adjustment of the "read" voltage-controlled oscillator.